In particular, the invention relates to tunable lasers for the mid infrared range (MIR). They may be used for spectroscopy, in particular. Examples: respiritory gas analysis (CO2, nitrogen, etc.), foodstuff analysis (molds, protein content, fat content, etc.), environmental measurement engineering (fine dust, pollution by hydrocarbons, etc.), detection of explosives, process analytics in the chemical and pharmaceutical industries.
The wavelength of the laser is changed step by step or continuously. The emitted laser beam passes through the material to be analyzed (transmission/absorption spectroscopy) or is reflected by same (reflection spectroscopy). The intensity of the resulting signal is determined by means of a detector as a function of the wavelength. The spectrum thus obtained serves as a basis for determining the composition of the material examined.
Tuning has to be performed either continuously or quasi-continuously, i.e. in steps sufficiently small so that a suitably high spectral resolution is achieved. Otherwise, important absorption lines characteristic for the material would not be dissolved or not sufficiently dissolved, whereby identification would be made more difficult or be prevented.
Depending on the substance to be examined, specific wavelength sections within the MIR are of interest.
In this context, the requirement arises to provide a miniaturized, continuously tunable MIR laser source which can be flexibly adjusted to the wavelength range that may be used. Further requirements result from the necessity to also examine dynamic processes, which involves fast tunability, and from the necessity to achieve a high degree of optical coupling efficiency, which involves that the resonator be sealed off by a mirror or a diffraction grating having suitably large dimensions.
A first known approach relates to using light sources with broad-band emissions.
In the MIR, light sources with broad-band emissions such as a Globar, for example, which are combined with a wavelength-selective element for spectroscopy, have sometimes been employed. Such a wavelength-selective element provides a tunable Fabry-Perot filter, for example. Alternatively, the broad-band light sources are combined with a Fourier transformation spectrometer.
When using a Fabry-Perot filter one has to take care to ensure that the achievable resolution of the filter and the free spectral range are strictly correlated. The broad tuning range that may be used for achieving flexible applicability is consequently only achievable with a corresponding significant reduction of the resolution. In order to achieve both a broad tuning range and a high resolution, a combination of several filters having different free spectral ranges is useful, which increases complexity and manufacturing cost of the systems.
To achieve tunability, the filter and the highly reflecting mirror generally may be offset in relation to one another—or the array is rotated with regard to the incident beam. This actuation nowadays is mainly performed by precision-mechanically produced motors, e.g. stepper motors. Thus, the degree of miniaturization is clearly limited. Alternatively, MEMS Fabry-Perot filters (MEMS=microelectromechanical system) might be employed, e.g. by Axsun Technologies, however only for the near infrared range (NIR). In this context, a high degree of miniaturization is achieved. However, the MEMS-based production involves using selected layers for producing the highly reflecting Bragg reflectors. While typical combinations of highdiffraction and low-diffraction layers can easily be made available in MEMS technologies in the NIR, materials such as CaF2 or other non-standard materials would be used in the MIR.
Due to the useful modulation of the optical path length, Fourier transformation spectrometers are generally slow and heavy and not portable since they are highly sensitive to vibrations. Even if first MEMS-based approaches are very promising in this area, the general problems that cooled detectors may be used for the MIR remains. Said detectors either have a very high electric power consumption (thermoelectric cooling module, Peltier element) or may be cooled by means of liquid nitrogen.
A further known approach relates to using light sources with narrow-band emission.
An alternative approach is the use of light sources with narrow-band emission, i.e. lasers, wherein the emitting wavelength may be changed by a suitable optical element. In the MIR, quantum cascade lasers have become widely accepted since other types of lasers actually do not cover this wavelength range. For tuning the emitted wavelength, wavelength-selective optical elements which limit the external laser cavity and/or are arranged within the external laser cavity are typically employed outside the laser chip.
As a first example of such an optical element, a Fabry-Perot filter is to be mentioned, which was already described above. FIG. 1 shows the fundamental arrangement with a quantum cascade laser chip, a lens for beam collimation, two tunable (Fabry Perot) filters and a highly reflecting mirror which seals off the external cavity. The first mirror of the cavity is formed by the side face of the laser diode (in FIG. 1, this is the left-hand side of the laser diode). To expand tunability, two Fabry-Perot filters have been cascaded, as was already described above.
Only those wavelengths which are transmitted by the two filters are amplified within the resonator and are thus emitted by the laser array. The fundamental disadvantages discussed above which result from using Fabry-Perot filters still remain.
An alternative approach is depicted in FIG. 2. Here, a diffraction grating is employed which performs the function of the Fabry-Perot filter and of the highly reflecting mirror.
The various wavelengths emitted by the laser chip impinge upon the diffraction grating. Depending on the grating period and the angle of incidence a, which may be varied by rotating the grating, in the first-order diffraction, only one wavelength is diffracted such that it moves back to the laser chip in a manner that is collinear with the incident beam. Said wavelength is amplified accordingly, so that the arrangement will emit at this wavelength. For this wavelength, the diffraction grating and the second highly reflecting mirror, which seals off the laser resonator, act as a Fabry-Perot filter. The filter and the wavelength diffracted back into the resonator by the diffracting grating may be tuned to each other. However, they change their properties as the angle of rotation of the grating changes in accordance with different angular functions. Thus, if the grating is rotated and if a different wavelength is thus diffracted back, the resonator length may be adapted to avoid mode hopping. The pivot point of the diffraction grating may be determined and adjusted individually for each configuration (grating period, central wavelength of the tuning range), so that a continuous tuning range is enabled. Simple flexible re-configuration would involve a pivot point that can be changed in a simple and automated manner. In technical solutions known today, mainly precision-mechanically produced motors are employed for rotating the diffraction grating, which motors may be re-mounted, however, in order to change the position of the pivot point. In addition to the lack of direct flexibility in terms of different wavelength ranges, miniaturization is not an option.
The variant shown in FIG. 3, wherein the diffraction grating is stationary, and a mirror is rotable instead, is very similar. By means of this so-called Littmann configuration one achieves that the resonator length—and, thus, the wavelength defined by the Fabry-Perot filter effect of the resonator—and the wavelength diffracted back by the tilting of the mirror change with the same angular function. A suitable pivot point is to be selected for this purpose, which may then remain stationary, however. This pivot point is typically located clearly outside the grating structure.
Examples of miniaturized architectures in MEMS technology for tunable laser sources within the visible range (VIS) and the NIR of up to 2 μm have been described, e.g., by A. Q. Liu et al. (A. Q. Liu et al.: “A review of MEMS external-cavity tunable lasers”, J. Micromech. Microeng. 17 (2007), RI-R13, and A. Q. Liu et al.: “Tunable laser using micromachined grating with continuous wavelength tuning”, Appl. Physics Letters, Vol. 85 (2004), pp. 3684-6). In one case, the grating and the electrostatic actuator for rotating the diffraction grating are produced by means of deep reactive ion etching (DRIE) in silicon. The diffraction grating has a restricted dimension in the vertical direction, which is due to the etching technique. Typical values here are 100 μm. As a result, the coupling efficiency is clearly restricted, especially in highly divergent laser diodes. Additional optical components such as cylinder lenses, for example, would be useful for achieving a high degree of efficiency. In deep reactive ion etching, the surface quality of the side walls is limited due to so-called scalloping (scoring caused by sequential etching and passivating in the DRIE process). This also has a negative effect on the coupling efficiency. Such an effect incidentally occurs in all DRIE etched structures, so that a mirror surface produced in this manner exhibits the same disadvantage. Finally, it is to be noted for the case of a diffraction grating etched by means of DRIE that the grating structure defined by the etching mask differs from the structure present at the surface to an increasing degree as the depth increases. This, too, has a negative effect on the grating quality and, thus, on the coupling efficiency.
The fundamental disadvantages of a DRIE etched structure also apply to the example given by W. Huang (W. Huang et al.: “Precision MEMS flexure mount for a Littman tunable external cavity laser”, IEE Proc.-Sci. Meas. Technol. Vol 151(2) (2004), pp. 67-75). Here, a mirror that can be tilted via electrostatic actuators is described, the actuators and the mirror surface having been produced by means of DRIE. As a result of complex solid-body suspension and different actuators for tilting and translation, the virtual pivot point of the mirror may be located far off outside the chip, so that the chip can be kept small. Both movements, however, are not mutually independent and thus involve complex control. The achievable tilting angle of approx. 0.1° is relatively small but sufficient for a tuning range of +/−50 nm. The length of the mirror is 2.3 mm, the height is only 75 μm for the above-mentioned reasons.
In order to compensate for the small height of the DRIE mirror and/or of the DRIE diffraction grating, cylinder lenses are used, for example. The requirement placed upon the quality of the lenses and the expenditure for adjustment may be reduced as the dimension of the mirror and/or of the diffraction grating increases, which is why mirrors/diffraction gratings having lengths and heights clearly larger than 100 μm are advantageous. In addition, the grating structures that may be used for the MIR scale with the wavelength, which is why larger refraction gratings/mirrors may be used here than would be the case for the NIR.
A different approach to compensating for the change in the resonator length upon rotation of the grating in the Littrow configuration consists in using precision-mechanical piezoelectric actuators for translational displacement of the grating or of the resonator mirror.